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To answer your first question. An active center diff locks up the differential at will with some form of clutch arrangement. When the clutch is closed it will be like having no diff at all. When open, it will behave like a standard open differential.

The benefit from this is that the engine torque will always go where there is grip when the diff is locked. For instance, if you had your back wheels on ice and front wheels on tarmac and an open diff, your rear wheels would spin and you'd go nowhere. If you could lock the diff all of the torque would go to the front wheels and you'd move forwards.

It is the same concept in the WRC cars. Under acceleration the diff would lock up and you'd gain more torque on rear wheels due to the front wheels lifting from acceleration (therefore the rear wheels having better grip).

I am not in any way an expert in how they make their diff maps and based on what sensors, angles, positions etc etc... That's for the real brains to work out but at least I gave you one scenario where it would be beneficial.

To answer your first question. An active center diff locks up the differential at will with some form of clutch arrangement. When the clutch is closed it will be like having no diff at all. When open, it will behave like a standard open differential.

The benefit from this is that the engine torque will always go where there is grip when the diff is locked. For instance, if you had your back wheels on ice and front wheels on tarmac and an open diff, your rear wheels would spin and you'd go nowhere. If you could lock the diff all of the torque would go to the front wheels and you'd move forwards.

It is the same concept in the WRC cars. Under acceleration the diff would lock up and you'd gain more torque on rear wheels due to the front wheels lifting from acceleration (therefore the rear wheels having better grip).

I am not in any way an expert in how they make their diff maps and based on what sensors, angles, positions etc etc... That's for the real brains to work out but at least I gave you one scenario where it would be beneficial.

Hello Itix

Everything you have described can be done - and is done - with a passive diff.
The potential in the active diff (an active centre diff in the WRC17) is that you can regulate power transfer any way you want, any time you want, on the background of the mapping.

So with these new active centre diffs the engineers can map the diffs to distribute power between front and rear as they want. And they will use certain sensors to get the raw data (speed, rotation, gear, throttle, braking, etc), and they will map the diff to distribute power in a certain way according to the parameters they set, and the "numbers" they get from the sensors.

The advatage of the active diff is that it is fully adjustable, so a driver can make the car suit him better in stead of battling it (if he has that kind of problem.) And the diff can react before its needed, so its ready from the second its needed, in stead of having the delay of the passive diffs.

There is also an option to switch maps from inside of the car (if it's allowed like with the old WRC cars) so in case of a sudden rain or any other change of conditons, with a puncture etc. the driver can change the diff mapping.

Everything you have described can be done - and is done - with a passive diff.
The potential in the active diff (an active centre diff in the WRC17) is that you can regulate power transfer any way you want, any time you want, on the background of the mapping.

So with these new active centre diffs the engineers can map the diffs to distribute power between front and rear as they want. And they will use certain sensors to get the raw data (speed, rotation, gear, throttle, braking, etc), and they will map the diff to distribute power in a certain way according to the parameters they set, and the "numbers" they get from the sensors.

The advatage of the active diff is that it is fully adjustable, so a driver can make the car suit him better in stead of battling it (if he has that kind of problem.) And the diff can react before its needed, so its ready from the second its needed, in stead of having the delay of the passive diffs.

Hi Lunde
So why is the Mitsubishi active center differential called like that? Because it is just a hydraulic clutch pack and nothing else. The active yaw control diff in the same case of Mitsubishi does pretty much what you describe (it's a torque vectoring differential) so you can in the case of the Mitsubishi send power to the right or left rear wheel as you wish.

Shouldn't we call it a torque vectoring differential rather than an active one in that case? I mean a passive differential is what I would call a limited slip differential. You can't really choose when to lock it and when not to.

There is also an option to switch maps from inside of the car (if it's allowed like with the old WRC cars) so in case of a sudden rain or any other change of conditons, with a puncture etc. the driver can change the diff mapping.

Hi Mirek (equal treatment to everyone )
Does that mean the old 99-2010 wrc cars had torque vectoring differentials too?

I remember they had very complicated hydraulic systems with accumulators and everything, and it failed quite often.

Hi to everybody, the old WRC's had active center differential and they have the option to switch between different mapping according to their need in the particular stage or even in the particular corner as Mirek said.
I think that the Mitsu WRC hasn't an active diff they were using a passive one if I remember correctly

Hi to everybody, the old WRC's had active center differential and they have the option to switch between different mapping according to their need in the particular stage or even in the particular corner as Mirek said.
I think that the Mitsu WRC hasn't an active diff they were using a passive one if I remember correctly

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This makes me even more confused... What would then be a passive center diff? To me it sounds like a standard limited slip diff pretty much.

Itix, the old WRC active center diffs could change the torque split ratio only indirectly by changing the slip level. The result was similar to what production Evos or Imprezas do but I believe they had more input channels and probably overall higher level of regulation. Dimviii or Br21 could tell You more for sure.

It would be interesting to know what is used in the new cars. I don't have a clue.

By the way have You ever played Richard Burns Rally? It features a simple active differential with a 3D map for settings - first axis is throttle, second axis is brake and third axis is vehicle speed.

Rage82, Mitsubishi Lancer WRC 05 had also active center diff. What it didn't have was active front and rear diff which was still allowed in 2005. The reason was that it was already known that since 2006 front and rear active diffs would have to be replaced with mechanical LSD ones. That's why they decided to use them already in 2005.